For a spread spectrum communication in its basic form, a data sequence is used by a transmitting unit to modulate a sinusoidal carrier, and then the bandwidth of the resulting signal is spread to a much larger value. For spreading the bandwidth, the single-frequency carrier can be multiplied for example by a high-rate binary pseudo-random noise (PN) code sequence comprising values of −1 and 1, which code sequence is known to a receiver. A PN code period comprises typically 1023 chips, the term chips being used to designate the bits of the code conveyed by the transmitted signal, as opposed to the bits of the data sequence.
Spreading codes are employed for instance, though not exclusively, in Global Navigation Satellite Systems (GNSS).
For the American GNSS GPS (Global Positioning System), for example, more than 20 satellites orbit the earth. Each of the satellites transmits two carrier signals L1 and L2. One of these carrier signals L1 has a frequency of 1575.42 MHz and is employed for carrying a navigation message and code signals of a standard positioning service (SPS). The L1 carrier phase is modulated by each satellite with a different C/A (Coarse Acquisition) code. Thus, different channels are obtained for the transmission by the different satellites. The C/A code is a pseudo-random noise (PN) code, which is spreading the spectrum over a nominal bandwidth of 20.46 MHz. It is repeated every 1023 bits, the epoch of the code being 1 ms. The carrier frequency of the L1 signal is further modulated with the navigation information at a bit rate of 50 bit/s.
A GPS receiver of which the position is to be determined receives the signals transmitted by the currently available satellites, and it detects and tracks the channels used by different satellites based on the different comprised C/A codes. For the acquisition and tracking of a satellite signal, a signal received by a radio frequency (RF) portion of the GPS receiver is first converted into the baseband. Then, the signal is sampled in an analog-to-digital (A/D) conversion, and the samples are correlated with the samples of replica codes that are available for all satellites. The correlation can be performed for example using a matched filter. A correlation value exceeding a threshold value indicates the C/A code and the code phase, which are required for dispreading the signal and thus to regain the navigation information.
While the GPS satellite signals thus comprise a data component, a PN component, and a sinusoidal carrier component, other spread spectrum signals may include an additional sub-carrier modulation.
For the European GNSS GALILEO, for example, four carrier signals E5a, E5b, E6 and L1 have been selected. The basic use of these carrier signals is similar as described for the GPS L1 carrier signal. The spectrum of the sinusoidal carrier signals E6 and L1, however, is spread using a binary offset carrier (BOC) modulation, which is composed of a BOC sub-carrier and a PN-code. This modulation is notated as BOC (n,m), where n denotes the chiprate of the PN-code and m the sub-carrier frequency in 1.023 MHz. The BOC sub-carrier will also be referred to simply as BOC carrier in the following.
To the GALILEO L1 carrier signal, a BOC(1,1) modulation has been assigned. FIG. 1 presents the modulation principle. A first diagram illustrates the progress of the BOC(1,1) carrier over time. The BOC(1,1) carrier consists of subchips having alternating values of +1 and −1, each subchip having a duration of 0.5 ms. A second diagram illustrates the progress of an exemplary PN-code over time. The PN-code consists of a sequence of chips having a value of +1 or −1, each chip having a duration of 1 ms. The beginning of each chip of the PN code coincides with a rising edge of the BOC(1,1) carrier. A third diagram illustrates the BOC(1,1) output signal, which is obtained by mixing the BOC(1,1) carrier with the PN-code, over time.
FIG. 2 is a diagram comparing a spreading of the carrier signal L1 using a GALILEO BOC(1,1) modulation and a GPS C/A code modulation. The diagram shows the amplitude of the spectrum of the modulated carrier in a range of −4 MHz to +4 MHz around the frequency of the carrier signal. It can be seen that with the BOC(1,1) modulation, the signal band is more than doubled compared to the C/A code modulation. With the BOC(1,1) modulation, most of the energy is concentrated in two main lobes on two sides of a single main lobe resulting with the C/A-code modulation. The side lobes resulting with the BOC(1,1) modulation are also strong when compared to those resulting with the C/A-code modulation. The energy distribution of the C/A code modulation and of the BOC(1,1) modulation is shown in the following table:
Main lobe(s)1st order lobes2nd order lodesC/A90%4.9% (2 × 2.4%)1.7% (2 × 0.84%)BOC(1, 1)86% (2 × 43%)7.1% (2 × 3.6%)2.7% (2 × 1.33%)
A BOC(1,1) signal acquisition can equally be realized by means of a matched filter. To this end, a received RF signal is downconverted and sampled. In order to exploit the signal energy to a large extent and to minimize the overlapping interference, the sampling rate should be sufficiently high to cover the first order side lobes. The minimum sampling rate should be 4 samples per chip (complex), which corresponds to 2 samples per subchip (complex). FIG. 3 is a diagram of a typical matched filter (MF) output resulting with such a BOC(1,1) signal.
It is a disadvantage of the signal acquisition based on a BOC(1,1) signal that the BOC(1,1) modulation doubles the required sampling rate and thus the number of required correlators. The higher sample rate is used throughout the acquisition chain, in order to limit the reduction of accuracy when the signal lies in between the replica code alignments of the correlators. As a result, the hardware complexity will be about four times higher than the hardware complexity for a non-BOC signal.
To the GALILEO E6 carrier signal, a BOC(10,5) modulation has been assigned. A BOC(10,5) modulation is an example of a BOC(n,m) modulation, with n=2m. For this type of BOC modulation, the relation between chips and subchips is the same as for BOC(2,1).
FIG. 4 presents the modulation principle for the representative BOC(2,1) modulation. A first diagram illustrates the progress of the BOC(2,1) carrier over time. The BOC(2,1) carrier consists of subchips having alternating values of +1 and −1, each subchip having a duration of 0.25 ms. A second diagram illustrates the progress of an exemplary PN-code over time. The PN-code consists of a sequence of chips having a value of +1 or −1, each chip having a duration of 1 ms. The beginning of each chip of the PN code coincides with a rising edge of the BOC(2,1) carrier. A third diagram illustrates the BOC(2,1) modulation over time, which is obtained by mixing the BOC(2,1) carrier with the PN-code.
FIG. 5 is a diagram comparing a spreading of the carrier signal L1 using a GALILEO BOC(2,1) modulation and a GPS C/A code modulation. The diagram shows the amplitude of the spectrum of the modulated carrier signal in a range of −8 MHz to +8 MHz around the frequency of the carrier signal. It can be seen that with the BOC(2,1) modulation, in order to cover the two main lobes of the BOC(2, 1) signal, the minimum bandwidth is ±3 MHz. The minimum usable sampling rate for BOC(2,1) is 4 MHz (complex) and if the first side lobes are also of interest, then a 8 MHz sampling rate should be used. With a sampling rate of 8 MHz, the required acquisition hardware complexity will obviously be even higher than with a sampling rate of 4 MHz.
It is to be understood that a relatively high hardware complexity may be required as well for the processing of other signals comprising a carrier or a sub-carrier that is modulated by a spreading code.
Since a BOC signal is a symmetric signal on both side of the carrier frequency, dual-band receiver structures could be used in order to reduce the bandwidth that is to be processed by the matched filter or by correlators. A corresponding acquisition is illustrated in FIG. 6. The symmetric signal 60 is provided on the one hand to an upper sideband filter 61 and on the other hand to a lower sideband filter 65. The upper sideband filter 61 provides the upper sideband signal to a downconverter 62 for downconversion. The downconverted signal is then provided to a first matched filter or to a first correlator bank 63. The lower sideband filter 65 provides the lower sideband signal to a second downconverter 66 for downconversion. The downconverted signal is then provided to a second matched filter or to a second correlator bank 67. The output of both matched filters or both correlator banks 63, 67 is then added by adder 69 to obtain the final correlation result.
This approach is efficient for a wide spread signal, such as a BOC(5,1) signal, since a narrow baseband signal can be achieved in the sideband filtering for each sideband. For a BOC(1,1) signal, however, for example, the benefit is not significant, since the two sidebands are close to each other, since the filter loss is a concern, and since the overlapping interference could be high in the Doppler environment.
Document EP 1 315 308 A1 presents a signal tracking unit, which utilizes an elimination of the sub-carrier of a BOC modulation, in order to render the tracking of the received radio signal more robust. The elimination is based on a multiplication of the sub-carrier with a replica of the sub-carrier in phase and in quadrature.
Still, the high sample rate prevents an efficient processing.